Functional Near Infrared Spectroscopy in Adult Surgery

Perioperative cerebral injury is a common and significant complication of patients undergoing cardiac surgery. 1,2 During cardiac surgery, cerebral injury can occur as a result of hypoperfusion, cerebral embolism, and/or inflammation. 3 These intraoperative cerebral injuries could manifest as postoperative neurocognitive dysfunction (e.g., impaired memory and attention, mood alterations), stroke, delirium, visual field deficits and coma. 4 Around 1% to 4% of cardiac surgical patients with closed-chamber cardiac procedures are complicated with stroke. The incidence of stroke increases markedly to about 8% to 9% in open-chamber (e.g., valvular surgery) or combined/complex procedures. These patients with neurological injury are associated with increased postoperative mortality, prolonged intensive care unit, hospital stay, decreased quality of life, and decreased long-term survival. 4 Despite the advancement of surgical techniques and perioperative care leading to a progressive decrease in cardiac surgical mortality since the 1980s1,5,6, the incidence of postoperative neurologic complications has remained relatively unchanged. This is largely related to the lack of effective monitoring and intervention to detect and mitigate intraoperative cerebral injury.

One potentially treatable cause of cerebral injury during cardiac surgery is hypoperfusion related infarct/ischemia. 7 Watershed, or boundary zone, are situated along border zones between the territories of two major cerebral arteries (e.g., the middle and posterior, or the anterior and middle cerebral arteries) where terminal arteriolar anastomoses exist which are more susceptible to hypoperfusion related infarct/ischemia. 8 These hypoperfusion related injuries are potentially treatable by increasing systematic blood pressure if those injuries are being identified early. Previous study has used electroencephalography to detect intraoperative brain injury at the watershed area9, but the clinical utility of electroencephalography is limited because of its cumbersome set-up and complex logistics. As such, this study aims to use a novel multichannel fNIRS to detect and monitor cerebral oxygenation at multiple brain regions simultaneously for adult patients undergoing during cardiac surgery. This study will increase the experiences of using fNIRS as a monitoring tool in cardiac surgery and improve the understandings to the cerebral injury pattern during cardiac surgery.

Multichannel functional near infrared spectroscopy (fNIRS) is an emerging brain-imaging technique to measure hemodynamic changes at the cortical surface. During the acquisition of fNIRS data light (600-900 nm) is emitted from light sources that can pass through the skull and reach cortex. Photons reflected from the cortex can be detected and quantified by detectors placed in close proximity to the sources. Light is mainly absorbed by oxygenated hemoglobin and deoxygenated hemoglobin, which have distinct absorption spectra. Alterations in the concentrations of oxy- and deoxy-hemoglobin derived from neuronal activation can be quantified using two or more wavelengths. The early studies has showed that the use of two channels cerebral near-infrared spectroscopy can avoid profound cerebral desaturation and is associated with significantly fewer incidences of major organ dysfunction. 10 However, one major limitation of two channels NIRS is that sensor placement is currently limited to forehead that only the frontal lobes are being monitored. Such placement prevents monitoring the critical posterior watershed at the juncture of the anterior, middle, and posterior cerebral arteries, and results in undetected stroke despite the patients having normal cerebral oxygenation throughout the surgery. The lack of comprehensive cortical coverage of the important brain regions such as water-shed area also limit the utility of two channels NIRS as a research tool to assess regional cerebral flow and metabolism during cardiac surgery. The multichannel fNIRS offers a number of advantages for assessing surgical patients intraoperatively including its high temporal resolution (~100ms), low-cost and portability. The multichannel fNIRS allows up to 64 channels of real-time data acquisition and interpretation, which allows adequate spatial and temporal resolutions to assess the brain regions at risk intraoperatively. In this study, only 16 channels will be used to assess the brain oxygenation.

The fNIRS system in this study is a class I Health Canada approved device (NIRx NIRScout 64x32 Laser/LED 4-wavelength fNRIS system, info:https://nirx.net/nirscout). It does not heat up nor can it damage eyesight. This commercially available device has been used safely in previous studies in neonatal population at Western (WREM #: 111168, 113443) and other studies published in the literature. This device is approved for brain oxygenation monitoring in humans.

Single site, prospective, observational study where patients undergoing cardiac surgical procedures will be monitored using multichannel fNIRS monitoring to assess cerebral oxygenation in regions of the brain that are susceptible to injury during cardiac surgery. No intervention will be used in this study. 40 patients total will be included in the study and receive this monitoring during surgery. There will be no randomization and no additional changes to standard of care.

Patients who agree to participate in this study will receive brain monitoring in the form of multichannel fNIRS monitoring that is not standard of care. This consists of wearing the NIRS cap that will be positioned on the patient's head according to international operational guidelines and will be secured to the temporal region of the patient's forehead. The NIRS cap is entirely non-invasive and does not pose any additional risk to the patient. Monitoring will be commenced prior to the induction of anesthesia.